First Input Delay

FID or My Uncle Bob's Reaction Time

Definition of First Input Delay

First Input Delay (FID) measures the time between when a user first interacts with a page (like clicking a button) and when the browser is able to respond to that interaction. It’s a key metric for assessing your site’s responsiveness.

Think of your Uncle Bob who falls asleep during conversations. You say something, and it takes a beat (or five) for him to respond. That’s FID on a poorly optimized site. Users expect your site to respond instantly, so a slow FID can frustrate them and damage your credibility.

FID as Part of an Observability Practice

Monitoring FID ensures that your users can interact with your site as soon as it’s visually ready. By tracking FID and fixing any slow responses, you build trust and create a frictionless user experience that encourages deeper engagement.